Java中String比较:==、equals()方法、intern()方法

- Java中String比较:==、equals()方法、intern()方法

Java中String比较:==、equals()方法、intern()方法

涉及到String的操作==,equals,intern方法。

*文中出现的代码基于JDK1.8_144版本.

一、 ==比较

1.01 简单类型

{ byte, boolean, short, char, int, long, float, double}

直接比较值。

1.02 引用类型

比较地址值，如果指向同一个地址，返回true。否则返回false。

1.03 String的==比较

可以对比2.01 String中的重写的equals()方法:来看。

废话不多说，直接上代码。

    /**
     * .
     * ==判断引用是否相等.
     */
    public static void testEquals2() {
        String a = "a";
        final String c = "a";

        String b = a + "b";
        String d = c + "b";
        String e = getA() + "b";

        String compare = "ab";

        System.out.println(b == compare);
        System.out.println(d == compare);//编译优化，final能确定，所以指向同一个
        System.out.println(e == compare);
    }

    /**
     * 测试用.
     * @return “a”
     */
    private static String getA() {
        return "a";
    }

由于编译优化机制，能在编译时确定的直接优化到常量池中。所以相等。如fianl。

-----testEquals2()-----
false
true
false

二、 euqals()比较

Object中的equals()方法与 == 相同。

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2.01 String中的重写的equals()方法:

Compares this string to the specified object. The result is true if and only if the argument is not null and is a String object that represents the same sequence of characters as this object.

比较他们的字符串值是否相同。

    public static void testEquals3() {
        String a = "a";
        final String c = "a";

        String b = a + "b";
        String d = c + "b";
        String e = getA() + "b";

        String compare = "ab";

        System.out.println(b.equals(compare));
        System.out.println(d.equals(compare));
        System.out.println(e.equals(compare));
    }

-----testEquals3()-----
true
true
true

毫无疑问相同，我们来看看JDK源码吧:

JDK8源码

    public boolean equals(Object var1) {
        if (this == var1) {
            return true;
        } else {
            if (var1 instanceof String) {
                String var2 = (String)var1;
                int var3 = this.value.length;
                if (var3 == var2.value.length) {
                    char[] var4 = this.value;
                    char[] var5 = var2.value;

                    for(int var6 = 0; var3-- != 0; ++var6) {
                        if (var4[var6] != var5[var6]) {
                            return false;
                        }
                    }

                    return true;
                }
            }

            return false;
        }
    }

*当然，String也重写了hashCode()方法。

三、 intern方法介绍

Returns a canonical representation for the string object.
A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.

All literal strings and string-valued constant expressions are interned. String literals are defined in section 3.10.5 of the The Java™ Language Specification.

intern()方法:
当调用intern方法时，JVM会在常量池中通过equals()方法来匹配是否存在等值的String,
如果存在，则直接返回其地址。否则再新建.
所以，只要值相等，用.intern()==.intern()判断一定相等.
但是intern效率不高.

    private static void testIntern() {
        String a = "a";
        String b = a + "b";
        String c = "ab";
        String d = new String(b);

        System.out.println(b == c);
        System.out.println(c == d);
        System.out.println(c == d.intern());
        System.out.println(b.intern() == d.intern());
    }

-----testIntern()-----
false
false
true
true

参考资料:

Java 8 API
< Java特种兵(上册) > 谢宇 ISBN: 9787121239359